Referring to FIG. 1 of the drawings, the reference numeral 100 generally designates conventional input circuitry for an analog-to-digital converter (ADC). Circuit 100 is generally described in co-pending patent application Ser. No. 12/199,804, which is hereby incorporated by reference for all purposes. As shown, circuit 100 generally comprises a transmission medium (represented by inductor L1), buffer 102 and sample-and-hold (S/H) circuit 104. Buffer 102 generally comprises NMOS transistor Q1 (which is coupled to inductor L1), cascoded NMOS transistors Q2 and Q3 (which receive bias voltages NCAS and NBIAS at their respective gates), and capacitor C1. S/H circuit 102 is generally represented (for the sake of simplicity of explanation) as resistor RS, sampling switch SS, and sampling capacitor CS.
With many conventional buffers, the “off-chip” driver that drives transistor Q1 (which is generally a source follower) can drive (moderate) continuous time loads but not switching loads (which are present with the use of S/H circuit 104). In this configuration, capacitor C1 (which is generally the same size as capacitor CS) provides a signal current for the load (by way of the transistor Q2), while the source follower (transistor Q1) is able to generally define the voltage for the switching load (S/H circuit 104). It is desired to keep the capacitor C1 small as it loads the “off-chip” driver. This will mean that transistor Q1 will also provide some portion of signal current. This configuration, with this additional constraint of small capacitor C1, can not only be problematic with respect to its inability to drive switching load but can also significantly and adversely impact the spurious free dynamic range (SFDR) of an interleaved ADC when used in an interleaved ADC, which uses many buffers to drive multiple S/H circuits. Therefore, there is a need for a circuit with improved performance.
Some other conventional circuits are: U.S. Pat. No. 4,634,993; U.S. Pat. No. 5,764,175; U.S. Pat. No. 5,872,469; U.S. Pat. No. 6,255,865; U.S. Pat. No. 7,385,427; U.S. Pre-Grant Publ. No. 20090206885; and PCT Publ. No. WO07/093,475.